One thing that's interesting to know is that the disease load in Europe actually went up considerably over time until the 20th century. It wasn't a constant; people in the 16th century CE had a lot more to contend with than those in the 3rd BCE. Pathogens tended to arrive, and then stick around, especially if they weren't so virulent that they burnt themselves out, or had animal reservoirs.
There are a few nasty bugs we see described in various ancient sources which may have died out in the interval, or could be identified with pathogens known to us, and it's the sort of thing historians of a certain stripe argue about. But they're greatly outnumbered by the number of really nasty diseases which had not arrived yet; smallpox, plague, cholera, typhus, probably measles, and others. Even malaria is probably not yet in the swamps around Rome right now.
This is not an incidental or insignificant thing, especially when you're thinking about how deaths from disease amongst armies seem to be less than you'd expect in Classical and Roman accounts. Not all of it was that the Romans knew how to dig latrine pits in straight lines.
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